1. Field of the Invention
The present invention relates to an engine for a motorcycle (motorcycle engine) improving an arrangement of various rotation shafts in an engine case.
2. Related Art
Prior arts provide, for example, Japanese Unexamined Patent Application Publication No. 2002-122028 (patent publication 1) and No. 2004-360773 (patent publication 2), which disclose structures for arranging rotation shafts, such as a crankshaft, a transmission input shaft, a transmission output shaft, and a balancer shaft (i.e., secondary balancer shaft), in an engine case of a motorcycle engine.
In the engine disclosed in the patent publication 1, the engine case is split into three sections, namely, an upper case section, a middle case section, and a lower case section. A crankshaft and a secondary balancer shaft are rotatably supported at a mating or joint surface between the upper case and the middle case, and a transmission input shaft and a transmission output shaft are rotatably supported at a mating surface between the middle case section and the lower case section. The transmission input shaft is disposed behind the crankshaft.
In the engine disclosed in the patent publication 2, the engine case is also split into two sections, namely, an upper case section and a lower case section. A crankshaft and a transmission input shaft are rotatably supported at a mating surface between the upper case section and the lower case section, and a transmission output shaft and a secondary balancer shaft are rotatably supported in the lower case section.
However, in the engine disclosed in the patent publication 1, two rotation shafts are disposed at each of the two mating surfaces between the upper case section and the middle case section and between the middle case section and the lower case section, and these four rotation shafts are arranged in the front-rear direction of the engine case. Therefore, the engine case becomes lengthened in the front-rear direction (longitudinal direction of the motorcycle), and the total size of the engine is thereby increased. Moreover, since the engine case has a three-splittable structure, the workability thereof and mountability of the rotation shafts are not good.
In the engine disclosed in the patent publication 2, the transmission output shaft and the secondary balancer shaft are disposed on the front and rear sides of the crankshaft and the transmission input shaft that are rotatably supported at the mating surface between the upper case section and the lower case section, that is, these four rotation shafts are arranged in the front-rear direction of the engine. This increases the total size of the engine as likely in the above-described engine.